Begonias prone to powdery mildew

August 10, 2007

HORTISCOPE---HAS STANDING HEAD By RON SMITH Extension Horticulturist North Dakota State University Q I have a begonia that has just gone through a long blooming period. However, there is a sticky, powdery, white substance all over the plant. Could you give me an idea of what might be wrong with it? I keep it indoors. I am thinking one of my problems is that it is getting too much light. (St. Cloud, Minn.) A This is probably powdery mildew, which is common on begonias. This shows up when the humidity gets high. A fungicide that is specific for controlling this fungus will prevent further spread. Make sure the fungicide is labeled for use on begonias. Q Can you plant a pee gee hydrangea in full sun or should it have some protection? Also, I have something eating the leaves on my Japanese tree lilac. What should I spray it with? Thanks for all your help. (e-mail reference) A Not knowing where on earth you live, I would suggest planting the hydrangea in light shade. They are pretty adaptable plants. Whatever was eating the leaves of the Japanese tree lilac is probably gone by now. You can, if you wish, use Bayer's Advanced Tree and Shrub Insecticide. When or if that critter or any other returns, that will be its last meal. Q I bought a plum tree about six years ago from the local farm store. I planted it in a spot that gets full sun. I do not feed it, but it has grown into a very pretty tree. However, it never has bloomed. I have removed suckers and even trimmed the top a little. Will it be a nonblooming, fruitless old maid? (e-mail reference) A It should come into bloom unless it is in the shade of a large tree. If you are not fertilizing it, there should be little to no soft, succulent growth, which sometimes keeps it in a vegetative state. Q I was wondering if you could help me with a question about my cedar trees. They have grown little buds all over. There are so many that it is weighing down the branches and the trees look terrible. What should be done? (e-mail reference) A You can do some light pruning at this time of year. Q I listen to you on North Dakota Public Radio and appreciate your excellent advice. I have a question for you. Is it possible to plant a hibiscus tree/shrub in North Dakota and have it survive the winter? My mother has a hibiscus that she has had for more than a year, but it is getting quite large. She brought it into the house and it bloomed all winter. This year, it is getting a bit unwieldy for the house, so she is wondering if she can plant it outside and cover it with leaves or straw in the fall. Is there any chance it would go dormant and survive the winter? Thank you in advance for your time. (e-mail reference) A Thanks for being a regular listener to North Dakota Public Radio and for the nice comments about the program! Hibiscus will not survive our winters unless we make a quantum leap in global warming. The best thing to do is keep it pruned within normal handling bounds. Keep the plant outside during the summer, but bring it inside before killing frosts arrive. If possible, put the plant by a bright window. Sometimes supplemental lighting is needed because of our long, dark winters. Q I have a question about a red maple tree that I have. The trunk is wet with some sort of sap that is drawing flies and butterflies. Can you help? (St. Clair Shores, Mich.) A What you are seeing is sap flow from insects or mites feeding on the foliage and excreting "honeydew" that is falling on the trunk. The problem also could be caused by borers. If that is the case, you would see sawdust in the area. Look into the foliage of the tree and along the branches to see if you can spot insects or mites. You might take a branch to a local garden center where they have competent people to help nail this down for you and make the proper recommendations for control. Eastern North Dakota has been plagued by maple cottony cushion scale, which has been causing exactly what you describe. Look for what appears to be little cotton dabs along the branch and on some of the foliage. If that is the case, the local garden store can sell you a systemic insecticide that effectively will control this and any other plant feeding pests. Q I hope your summer is going well. I am writing about our raspberry patch. Many of the fruit- bearing canes are wilting and drying up. We did have 4 inches of rain several weeks ago, so the patch sat in water for a day or so. The water used to drain away, but the rows have become sort of dams and keep the water from running off. I installed an irrigation system last year, so I may have been killing them with kindness. On the Web, I saw a photo that looks just like what I see in my patch. I believe it is called phytophthora, which distills down to root rot. Does that seem right to you? The berries shrink on the withering canes and are useless. This is mostly occurring in the lower areas of the patch where the water sat the longest. The problem isn't affecting the new canes. Also, in yesterday's 95-degree heat, the mature berries on the good canes seem to sunscald on the south side of the berry. Is that related to the root rot or can the sun affect the berry that way? I have not seen it happen in our patch before. If the sunscald is severe, it ruins the berry. If the sunscald is light, the berries taste OK. Thanks for your help and for the ongoing column. (e-mail reference) A The berries can get scalded in hot, sunny weather, especially when they have grown soft from the rain and irrigation. Generally, once a berry patch gets established, little to no irrigation is needed unless a very dry, almost rainless spring shows up. I never watered my raspberry patch in New York and don't in our small patch in Fargo. The symptoms do resemble root rot. The suggestion I would make is to get it completely dug out during a cool day this summer. I would cut back into healthy tissue to be sure to get it all out of the soil. Knowing raspberries as we both do, that gap will fill in quickly. Q We have a Canadian red cherry tree that we had moved in from our city tree bank about two years ago. About a week ago, we noticed some of the foliage had been destroyed or was full of holes, mostly on the lower branches. We do not see any sign of worms or insects. Any ideas what is causing this or suggestions of what to do? (e-mail reference) A This is usually caused by bacterial spot (Xanthomonas pruni) or shot-hole disease (Pseudomonas syringae), which are similar organisms. Unfortunately, there is no effective way to control the problem with chemicals, except with the possible use of a Bordeaux mixture. The best way to control the problem is by altering your cultural practices. Keep high-nitrogen fertilizer (typically lawn fertilizers) away from the tree and practice excellent sanitation. This involves picking up all fallen leaves, twigs and fruit. Spraying lime-sulfur in the early spring, while the tree is dormant, may be helpful. Q I have a dwarf orange tree planted in the backyard. It has been bearing fruit for years. This year, there is white fuzz where the orange connects to the branch. Some leaves are falling off the tree and are very oily. I do not see any crawling pests on the leaves or around the fruit. Is there a solution for this problem? (e-mail reference) A I assume you live somewhere in the south, such as Texas, Florida or Arizona, where you can grow such beautiful trees! This sounds like cottony cushion scale attacking your trees. Take a branch sample to the local Extension Service office to see if my diagnosis is correct. The Extension person can tell you what the acceptable and legal means of controlling the problem. You don't want any lowly insect taking away your pleasure of consuming those oranges when they are ripe. Q I have a dieffenbachia that is beautiful! I normally give the plant about two cups of room- temperature water every 12 days during the summer. I am very happy about the new shoots that are sprouting. However, since the last time I watered, it appears that the sprouts have stopped growing. The leaves are still rigid and there is no yellowing of the leaves. What would cause the sprouts to stop growing? (e-mail reference) A Plant growth is energy consuming, so when there is available energy within the plant, it grows. If there insufficient energy to produce new growth, it remains quiescent. Since we are in the middle of summer, try adding a diluted amount of houseplant fertilizer to see if that helps stimulate the new shoots to continue growing. Keep in mind that all green plants are superb chemists, so they are able to utilize sunshine as the primary source of energy to produce new growth. In any case, this is not something you need to worry about. Q I am hoping you can help answer a question about a chokecherry tree we have. It gets little, white blossoms in the spring, but it never has produced berries. Would you have any idea why it wouldn't be producing berries? I live in Wisconsin. (e-mail reference) A There are a number of reasons why a fruit tree fails to produce. Extreme temperatures in winter can kill flower buds, while late spring frosts can kill the staminate part of the flower. There may be a lack of pollinating insect activity, especially bees. The temperatures may have been too low, wind too high or there was a rainy period when the pollen was ripe, so adequate pollination could not take place. There could have been overfertilization of the tree, either intentionally or as a side effect from lawn fertilization. The fruit tree may be too close to a large shade tree. Generally, these trees are prolific producers of fruit that is relished by birds for food and by us humans for wine and jelly making. Q I have a crown of thorns that is a very healthy, happy plant. It constantly is growing new leaves and it flowers through the year. My only disappointment is that there is no side growth. The plant is a single spike. Is there any way to encourage side growth? I've done a bit of research, but I'm unable to find a clear solution. One Web site suggested that I prune up to two-thirds of the growth. Is this safe for the plant? (e-mail reference) A The plant can be pruned without causing problems, but I wouldn't advise cutting back that much of the plant. I would suggest that about 25 percent to 30 percent of the growth be cut back. You should see some good branching following the pruning. Q I was walking through Whitman Mission National Historic Park a couple of weeks ago and walked directly under a huge willow tree that was growing along the former path of the Walla Walla River bed. While standing there, I felt a cooling mist on my skin. As I looked up to see where the mist was falling from, I saw a shower of very fine water droplets falling from the sky above the tree. Another couple walking by stopped and also noted the phenomenon. The man commented that it was probably aphid droppings, but it wasn't sticky and we saw absolutely no bugs on any of the leaves. Is it possible that this mist is the reason the weeping willow got its name? I would hate to think that it was really bug juice because it was so cooling and pleasant as I stood under that gigantic tree. (e-mail reference) A If it wasn't bug juice, then it was likely water vapor coming from small openings in the ends of the leaves called hydathodes. This often happens when the tree has an extensive root system. The tree can mine water from the soil. A strong hydraulic pressure can be created in the soil if there is soil moisture present, a high air temperature and relatively low humidity in the atmosphere. It also sometimes happens after extensive irrigation cycles or major rain events. The presence of hydathodes is not limited to any particular species of deciduous tree. Your interpretation of the "weeping willow" is a very nice one, but I'm certain that it refers to the architectural form the tree takes. More commonly, there are aphids, scale or spider mites that are causing the misting. However, everything around and directly under the tree would be sticky, which is something you would have noted while walking beneath the tree.